how can creationists justify their belief that all that we see was made 6000 or so years ago?
That's easy.
First you call the person asking the question a secular materialistic, atheist, liar and then you ask him what a super nova smells like. Next you point out that he could not possibly know how stars age and explode in the early universe because he was not there.
To complete the recipe, you read a couple of verses of Genesis into the record.
Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
I have never met a man so ignorant that I couldn't learn something from him. Galileo Galilei
If there is no struggle, there is no progress. Those who profess to favor freedom, and deprecate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightning. Frederick Douglass